1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same and, more particularly, to a semiconductor device in which an active layer is formed on an insulating substrate and to a method of manufacturing this device.
2. Related Background Art
Recently, in association with very large and large scale integration of semiconductor elements, complete electrical isolation among elements and the reduction in stray capacitance are important subjects of consideration. In addition, due to development of a long or large-area image device, making of a long or large-area active element and the like are the significant subjects of consideration. To cope with these subjects, various kinds of researches have been made with respect to the technology to form a semiconductor thin film crystal on various insulating substrates (for example, SOI (Silicon on Insulator) technology) and semiconductor devices using this technology.
Recently, a semiconductor device has been realized in which an active layer is formed on an amorphous insulating substrate such as SiO.sub.2 or the like is particularly required for the purpose of application to a three-dimensional integrated circuit having a multilayer structure, plane liquid crystal display device, long line sensor, or the like. As a material to be used for such an active layer, for example, amorphous silicon, polycrystalline silicon, and silicon which was monocrystallized due to a melting recrystallization (hereinafter, this silicon is referred to as a "pseudo monocrystalline silicon") have been studied. In general, three states of amorphous, polycrystal, and pseudo monocrystal are determined by the forming temperature of the material. In the case of forming silicon on SiO.sub.2, it becomes amorphous at temperatures below a crystallization temperature T.sub.c (about 500.degree. C.), pseudo monocrystal at temperatures above a melting point T.sub.m (1420.degree. C.), and polycrystal at temperatures within a temperature range from about the crystallization temperature T.sub.c to the melting point T.sub.m.
After a semiconductor layer is first deposited on an insulating substrate, it is heated to temperatures above the melting point T.sub.m and then recrystallized due to a solidification cooling, thereby forming a pseudo monocrystalline semiconductor layer. Thus, the polycrystalline or monocrystalline semiconductor layer of large diameter particles is formed and active elements such as transistors and the like can be formed on this semiconductor layer. In the case where transistors were formed, its carrier mobility is hundreds of cm.sup.2 /V.multidot.sec. This mobility is nearly equal to that of the transistors formed on a monocrystalline silicon.
However, according to such a method, in the case of recrystallizing the semiconductor layer deposited, the temperature needs to be set to a high temperature above the melting point T.sub.m (about 1420.degree. C. in the case of silicon), so that there are the problems such that the semiconductor layer is softened or in the worst case, the substrate itself is fused.
On the other hand, in the case of manufacturing an integrated circuit having a multilayer structure such as a three-dimensional integrated circuit or the like, according to a method whereby the heat treatment at such a high temperature is necessary, there is also the problem such that the impurity profiles of the elements which have already been formed in the lower layer portion are changed due to the high temperature, so that it is difficult to realize desired characteristics.
On the contrary, a low pressure chemical vapor phase (LPCVD) method, an MBE method, and the like have been known as methods of forming a semiconductor layer at relatively low temperatures below the melting point T.sub.m. In this case, an amorphous or polycrstalline semiconductor layer is formed as mentioned above.
In the case of the amorphous semiconductor layer, however, since the long distance order of the crystal structure is lacking, a carrier mobility of transistors formed on this layer is below 1 cm.sup.2 /V.multidot.sec. Thus, high speed operation characteristics cannot be realized.
On the other hand, in the case of the polycrystalline semiconductor layer, because of the diffusion of carriers mainly due to the crystal grain boundary, a carrier mobility of transistors formed on this layer is less than 10 cm.sup.2 /V.multidot.sec. Therefore, this semiconductor layer is still insufficient when it is used as an active element for various kinds of devices.